中国黒河下流域における最近3000年間の沙漠環境の変遷と人間活動

摘要

Change of desert environment and migration of rivers, deltas and terminal lakes in the lowest reaches of Heihe River during the last 3000 years was investigated mainly by field observation, micro-fossil and chemical analyses of sediments, AMS datings and satellite images, in relation to the shift of delta and sand dunes, and human activities.The extension of Old Juyan Lake in the past was reconstructed by the former shorelines (gravel bars) and those ages were determined by AMS method using molluscan fossils from the bar deposits. Datings of the former shoreline and lake bottom deposits indicate that a large Old Juyan Lake was existed from 7500 to 1700 years BP just faced on the Juyan Delta. Around the Green City and western half of Juyan Delta, human occupation including irrigation channels and agricultural land was dominated at least in Han Dynasty.After AD 250, water area of the Old Juyan Lake was decreased quickly. At this moment the main stream began to enter into Sogonur. Sand dune formation was activated and disturbed the river course. However a tributary stream already entered into Sogonur before 2100 BP.Around AD 1200, Heihe channels migrated to northward, and commenced to form Ejina Delta. Frequent flooding occurred and the river water reached to Gashun Nur. In Xixia and Yuan Dynasties, they used still irrigation channels for agriculture around Black City and in the western part of Juyan Delta.During the term between1400 to 1900 AD, corresponding to the Little Ice Age, Gashun Nur environment alternated between water coverage and desiccation based on the data of pollen and diatom analyses. AD 1450 - 1550, irrigation channels were covered with Tamarix cones, suggesting abandonment of the human occupation under such arid environments in the lower reaches of Heihe.